Forsyth reported on his early activities as a journalist. His early career was spent covering French affairs and the attempted assassination of Charles de Gaulle. He had never been to what he termed "black Africa" until reporting on the Nigerian Civil War between Biafra and Nigeria as a BBC correspondent.[7]
He was there for the first six months of 1967, but few expected the war to last very long considering the poor weaponry and preparation of the Biafrans when compared to the British-armed Nigerians. After his six months were over, however, Forsyth – eager to carry on reporting – approached the BBC to ask if he could have more time there. He noted their response:

I was told quite bluntly, then, 'it is not our policy to cover this war.' This was a period when the Vietnam War was front-page headlines almost every day, regarded broadly as an American cock-up, and this particularly British cock-up in Nigeria was not going to be covered. I smelt news management. I don't like news management. So I made a private vow to myself: 'you may, gentlemen, not be covering it, but I'm going to cover it.' So I quit and flew out there, and stayed there for most of the next two years.

He thus returned to Biafra as a freelance reporter, writing his first book, The Biafra Story, in 1969.[8]

In August 2015 Forsyth revealed that in Biafra he began work as a spy for MI6, a relationship that continued for 20 years. He claimed he wasn't paid.[9]

He is an occasional radio broadcaster on political issues, and has also written for newspapers throughout his career, including a weekly page in the Daily Express. In 2003, he criticised "gay-bashers in the churches" in The Guardian newspaper.[10] He has narrated several documentaries, including Jesus Christ Airlines, Soldiers: A History of Men in Battle and I Have Never Forgotten You: The Life & Legacy of Simon Wiesenthal.

In Forsyth's second full-length novel, The Odessa File (1972), a reporter attempts to track down an ex-Nazi SS officer in modern Germany. The reporter discovers him via the diary of a Jewish Holocaust survivor who committed suicide earlier, but he is being shielded by an organization that protects ex-Nazis, called ODESSA. This book was later made into a movie with the same name, starring Jon Voight, but there were substantial alterations. Many of the novel's readers assumed that ODESSA really existed, but historians disagree.[11]

In The Dogs of War (1974) a British mining executive hires a group of mercenaries to overthrow the government of an African country so that he can install a puppet regime that will allow him cheap access to a colossal platinum-ore reserve. This book was also adapted into the 1980 film starring Christopher Walken and Tom Berenger.

The Shepherd was an illustrated novella published in 1975. It tells of a nightmare journey by an RAF pilot while flying home for Christmas in the late 1950s. His attempts to find a rational explanation for his eventual rescue prove as troublesome as his experience.

Following this came The Devil's Alternative in 1979, which was set in 1982. In this book, the Soviet Union faces a disastrous grain harvest. The US is ready to help for some political and military concessions. A Politburo faction fight ensues. War is proposed as solution. Ukrainian freedom fighters complicate the situation later. In the end, a Swedish oil tanker built in Japan, a Russian airliner hijacked to West Berlin and various governments find themselves involved.

In 1982, No Comebacks, a collection of ten short stories, was published. Some of these stories had been written earlier. Many were set in the Republic of Ireland where Forsyth was living at the time. One of them, There Are No Snakes in Ireland, won him a second Edgar Allan Poe Award, this time for best short story.

Forsyth's tenth book came in 1989 with The Negotiator, in which the American President's son is kidnapped and one man's job is to negotiate his release.

Two years later, in 1991, The Deceiver was published. It includes four short stories reviewing the career of British secret agent Sam McCready. At the start of the novel, the Permanent Under-Secretary of State (PUSS) of the Foreign and Commonwealth Office requires the Chief of the SIS to push Sam into early retirement. The four stories are presented to a grievance committee in an attempt to allow Sam to stay on active duty with the SIS.

In 1999, Forsyth published The Phantom of Manhattan, a sequel to The Phantom of the Opera. It was intended as a departure from his usual genre; Forsyth's explanation was that "I had done mercenaries, assassins, Nazis, murderers, terrorists, special forces soldiers, fighter pilots, you name it, and I got to think, could I actually write about the human heart?"[12] However, it did not achieve the same success as his other novels, and he subsequently returned to modern-day thrillers.

In 2001, The Veteran, another collection of short stories, was published, followed by Avenger, published in September 2003, about a Canadian billionaire who hires a Vietnam veteran to bring his grandson's killer to the US. Avenger was released as a film starring Sam Elliott and Timothy Hutton.[13]

The Afghan, published in August 2006, is an indirect sequel to The Fist of God. Set in the very near future, the threat of a catastrophic assault on the West, discovered on a senior al-Qaeda member's computer, compels the leaders of the US and the UK to attempt a desperate gambit — to substitute a seasoned British operative, retired Col. Mike Martin (of The Fist of God), for an Afghan Taliban commander being held prisoner at Guantánamo Bay.

The Cobra, published in 2010, features some of the characters previously featured in Avenger, and has as its subject an attempt to destroy the world trade in cocaine.

On 20 August 2013, his novel, The Kill List was published. It was announced earlier in June that Rupert Sanders would be directing a film version of the story.[14]

On 10 September 2015, Forsyth's autobiography, The Outsider: My Life in Intrigue, was published.

In January 2018 it was announced that Forsyth would publish his eighteenth novel, a thriller about computer hackers, inspired by the Lauri Love and Gary McKinnon stories.[15] In October 2018, The Fox, was published in electronic format and is scheduled to be released in hardcover in November. The Fox is an espionage thriller about a gifted hacker.

In September 2005, Forsyth appeared on the ITV gameshow Who Wants To Be A Millionaire? and raised £250,000 for charity. On 8 February 2007, Forsyth appeared on BBC's political panel show Question Time; on it, he expressed scepticism on the subject of anthropogenic climate change. On 26 March 2008, he also appeared on BBC's The One Show. On 17 June 2008, Forsyth was interviewed on BBC Radio 5 Live Midday News in relation to the restoration of the Military Covenant. On 2 February 2015, he appeared on Eggheads as a member of Rewarding Talent.

In 2005, he came out in opposition to Kenneth Clarke's candidacy for the leadership of the Conservative Party, calling Clarke's record in government "unrivalled; a record of failure which at every level has never been matched". Instead, he endorsed and donated money to David Davis's campaign.

In 2016, he said he was giving up writing thrillers because his wife had told him he was too old to travel to dangerous places.[19]